It is known that the internal arrangement of the passenger cabin of an aircraft is dependent on certain technical installations necessary for the correct operation of the aircraft and for the comfort of the passengers and that run through its fuselage. Also, in order to create enough cabin space, these installations are attached along the walls of the fuselage and, in particular, in the upper part. In general, two longitudinal structural ducts spaced apart parallel to one another are supported by fixings at the upper parts of the walls of the fuselage, respectively on either side of the longitudinal plane of symmetry of the fuselage. These ducts accommodate the auxiliaries concerned, that is to say the installations relating in particular to the ventilation, illumination, audio-video equipment, oxygen, etc.
To cover these auxiliaries and other circuits and the walls of the fuselage an internal arrangement is provided which generally comprises:                a longitudinal central element forming a dome fixedly connecting the two parallel structural ducts;        a lower central ceiling panel situated under the dome and spaced away from it and facing toward the cabin; and        two lateral covering assemblies, each extending from the corresponding structural duct as far as the lateral side of the walls of the fuselage and being fixed to said duct and to said fuselage.        
Each assembly is therefore made up of three parts. A first part in the form of a curved lateral panel starting from the lateral side of the fuselage and extending as far as the structural duct and fixed to the walls of the fuselage and attached to the corresponding longitudinal edge of the duct concerned; a second part in the form of a flat panel fixedly attached under the structural duct in order to close it off and itself acting as a support for the auxiliaries; and a third part in the form of an edging strip fixed to the other longitudinal edge of said duct concerned and concealing the fixings and the connection between the longitudinal edges of the ceiling panel and the structural duct.
Although they are widely used, these arrangements do have certain disadvantages.
Because of the numerous screw or other connections and the parts involved in making up the arrangement, the sequences of removing and refitting in order to carry out maintenance or change parts of the auxiliaries are complicated, lengthy and involved. For example, in order to gain access to the auxiliaries in one of the structural ducts, it is necessary to remove at least the second and third parts (if not all three in some cases) of the covering assembly concerned, since the second part also acts as a wall of the structural duct. Likewise, in order to access the dome, which is fixed to said ducts, and work on the circuits and the like situated in the space between the dome, the corresponding structural duct and the walls of the fuselage involved, it is necessary to take off the edging strips and the ceiling panel in question, then remove the dome fixings in order then to extract it between the two ducts in a movement which is tricky because it is fixed to these ducts (lifting it then tilting it in order to make it pass between the ducts) and risky.
It will be understood therefore that the excessive number of parts (particularly each covering assembly) and their arrangement (connection of the assemblies with the ducts, dome over the ducts, etc.) contribute to the complexity of the arrangement and to making this arrangement cumbersome.
In addition, esthetically, the visual presence of longitudinal grooves between the parts of the assemblies is unbecoming, and this is detrimental to a specifically luxury arrangement of the cabins for privileged passengers (VIPs). In addition, the space available at the upper part of the cabin cannot be increased because of the fixed relationships between the assemblies and the structural ducts.